Systems for initiating the polymerization of monomers to make adhesive compositions are known in the art. U.S. Pat. Nos. 5,106,928, 5,286,821, and 5,310,835 to Skoultchi et al., for example, describe two-part initiator systems for initiating the polymerization of acrylic monomers. The first part of these two-part systems includes a stable organoborane amine complex and the second part includes an activator. The activator liberates the organoborane compound by removing the amine group, thereby allowing the organoborane compound to initiate the polymerization process. Activators are also sometimes referred to as liberators or decomplexers.
U.S. Pat. No. 5,286,821 to Skoultchi describes that suitable activators for liberating the organoborane compound include aldehydes having the general structure:R—(CHO)Xwherein R is an alkyl group having 1 to 10 carbon atoms or an aryl group having 6 to 10 carbon atoms and x is 1 to 2. Examples include benzaldehyde; o-, m-, p-nitrobenzaldehyde; 2,4-dichlorobenzaldehyde; p-tolylaldehyde; and 3-methoxy-4 hydroxybenzaldehyde.
U.S. Pat. Nos. 5,310,835 and 5,106,928 describes that suitable activators for liberating the organoborane compound also include organic acids having the structure:R—COOHwherein R is H, an alkyl group, or an alkenyl group having 1 to 8 carbon atoms. Examples include acrylic acid, methacrylic acid, benzoic acid, and p-methoxybenzoic acid.
Fujisawa, Imai, and Mashuhara also describe a system for initiating the polymerization of methyl methacrylate (See, Reports of the Institute for Medical and Dental Engineering, vol. 3, page 64 (1969)). The system comprises a trialkylborane amine complex and an activator such as the chloride of methacrylic or n-butane sulfonic acid, terephthalic acid chloride, benzoyl chloride, p-toluene sulfonic acid chloride, benzene sulfonic acid chloride, methane sulfonic acid chloride, toluene diisocyanate, adipic acid chloride, o-tolyl isocyanate, acetyl chloride, and acetic acid anhydride. The initiator system is reportedly useful in providing fast curing resins for dental applications.
One disadvantage of some of the aforementioned activators, however, is the relatively high odor of the activators and the composition in which they are used. Furthermore, another disadvantage of some of these activators is the relatively high level of mobile activator-amine reaction products (i.e., activator-amine constituents) in resulting adhesives. Typically, activators liberate the organoborane compound by bonding (either covalently or ionically) to the amine to form an activator-amine constituent. Most activator-amine constituents remain in the adhesive composition as mobile constituents that are not incorporated into the polymerized adhesive per se. In general, mobile constituents in adhesive compositions may cause performance problems when, for example, they migrate to the surface of the adhesive thereby disrupting the bond interface. Mobile constituents are also susceptible to attack by solvents, thereby making the adhesive composition less suitable for applications where exposure to solvents is unavoidable.
Recently, it has been disclosed that certain activators are useful for reducing the level of mobile constituents in adhesive compositions polymerized therewith. For example, PCT Publication No. WO 97/07171 discloses bireactive decomplexers preferably comprising at least one free-radically polymerizable group and at least one amine-reactive group, preferably an isocyanate group, in the same molecule. The bireactive decomplexers are capable of forming a covalent bond with both acrylic monomers and the amine portion of the organoborane amine complex. Accordingly, the decomplexer is capable of covalently bonding to the liberated amine in the system and also reacting into the adhesive per se.
PCT Publication No. WO 97/17383 discloses decomplexers comprising at least one anhydride group. Preferred anhydrides have one of the following structures: wherein R1 and R2 may independently be an aliphatic group; a cycloaliphatic group; or an aromatic group. R3 is a divalent organic radical that completes a cyclic structure with the anhydride group. It is further preferred that the decomplexer comprises at least one free radically polymerizable group that is capable of forming a covalent bond with an acrylic monomer, such that the decomplexer-amine reaction product is capable of forming covalent bonds with acrylic monomer(s) and can be incorporated into the polymerized adhesive per se.
PCT Publication No. WO 99/64475 reports that suitable activators for liberating the organoborane compound also include carboxylic acids having the structure represented below: wherein:
R1 is selected from the group consisting of hydrogen, a monovalent organic group, and a multivalent organic group;
R2 is a multivalent organic group;
R3 is selected from the group consisting of hydrogen and a monovalent organic group;
R is a monovalent organic group comprising an alkyl group having at least nine carbon atoms, preferably at least 15 carbon atoms;
m is an integer of 0-2; and
n is an integer greater than or equal to one.
The previously reported decomplexing agents including, for example, carboxylic acids, carboxylic acid chlorides, carboxylic acid anhydrides, aldehydes, isocyanates, and sulfonyl chlorides can be corrosive, irritating or hydrolytically unstable materials. In addition, these previously disclosed decomplexing agents generally react quickly with organoborane-amine complexes, thereby quickly freeing the organoborane for initiation of free radical polymerization. Rapid initiation of free radical polymerization may lead to an undesirably short worklife of the bonding composition. In view of the foregoing, what is needed is a substantially non-corrosive, non-irritating, storage stable decomplexing agent which preferably reacts slowly with an organoborane-amine complex, slowly freeing organoborane for initiation of free radical polymerization, and thus providing longer bonding composition worklife.